Conventionally, there are known rolling element screw devices in each of which a plurality of rolling elements are disposed between a screw side spiral groove formed in an outer peripheral surface of a screw member and a nut side spiral groove formed in an inner peripheral surface of a nut member so as to convert a thrust force of the screw member to a torque of the nut member or vice versa. These rolling element screw devices include one type provided with a retainer ring for retaining rolling elements between the screw member and the nut member so as to realize stable torque-thrust conversion by aligning the rolling elements by means of the retainer ring.
Operation of the rolling element screw device of the type using the retainer ring such as mentioned above will be explained hereunder. For example, when the nut member of the rolling element screw device is rotated, the rolling elements disposed between the nut member and the screw member roll and, simultaneously, the retainer ring having a retainer pockets in each of which the rolling element is accommodated is pushed by the rolling elements, so that the retainer ring is moved in an axial direction thereof while being rotated as like as the nut member in accordance with revolving speed of the rolling element.
During such operation, when the rolling element regularly rolls and any irregular sliding motion is not caused, the retainer ring is moved in the axial direction in accordance with revolution number and spiral groove lead while rotating at the same speed as theoretical revolution speed of the rolling element. However, the retainer ring has a nature of delay by a predetermined amount with respect to the rotational phase of the nut member. This is a known matter based on the rotation operation theory of the retainer ring in a general rolling bearing.
Of course, in an actual operation, there is a case where the rolling element does not regularly roll and irregular sliding motion is caused, and accordingly, the rotational phase of the retainer ring may advance or delay with respect to the ideal motion as mentioned above in response to the irregular sliding motion. Particularly, such advance amount or delay amount is different in the moving direction or moving amount in the axial direction of the nut member.
Because of this reason, when the nut member is rotated repeatedly reversely in directions, the advancing amount or delaying amount in the rotational phase of the retainer ring is accumulated and the retainer ring is sifted in either one side in the axial direction thereof. In the case when such one-side shifting is caused and progresses, the rolling element pocket of the retainer ring may protrude in one direction from a one shaft end side of the nut member, and as a result, there may cause a case that the rolling element drops off from the rolling element pocket of the retainer ring.
Then, in the conventional technology, for example, in the screw member, one stopper member such as pin or stopper ring for limiting movable range in the axial direction of the retainer ring is mounted to each of both end portions of a movable stroke range of the nut member.
However, in a structure in which the axial movable range of the retainer ring is limited by the stopper member, when the retainer ring abuts against the stopper member, the retainer ring is stopped from moving and the rolling element slides while rotating but not revolving, so that driving torque of the nut member extremely increases at this sliding time and the nut member is not smoothly moved, thus providing a problem.
Then, in order to solve such problem, a technology disclosed in the following Patent Document 1 has been proposed. That is, in the following Patent Document 1, there is proposed the technology in which, in a rolling element screw device of the type using a retainer ring, the one side shifting in the axial direction of the retainer ring is eliminated and the problem provided in the conventional technology is solved. More specifically, the technology disclosed in the following Patent Document 1 is characterized by a ball screw device using a retainer ring 104, as shown in FIG. 6, which is provided with a power transmission mechanism composed of an interior tooth 107 formed to a land portion adjacent to a spiral groove 101a of a nut member 101, an external tooth 108 formed, at an equal interval in the spiral direction, to a land portion adjacent to a spiral groove 102a of a nut member 102, and a planetary gear 109 held to be rotatably to a predetermined position in the circumferential direction of the retainer ring 104 in a state meshed respectively with the interior tooth 107 and the exterior tooth 108.
In the Patent Document 1, the power transmission mechanism of the structure mentioned above is provided for the ball screw device, and accordingly, the planetary gear 109 is meshed with the internal tooth 107 or external tooth 108 and moved in this state, so that the retainer ring 104 maintains a constant relationship with respect to rotational phase of the rotating side member (i.e., nut member 101 or screw member 102) to thereby always keep constant the moving amount in both the axial directions of the retainer ring 104. That is, according to the technology disclosed in the Patent Document 1, as means for limiting the axial movement of the retainer ring 104, a stopper member of the conventional structure is used to avoid such a phenomenon as that driving torque increases when the retainer ring abuts against the stopper member and to realize smooth operation of the device to thereby contribute to the improvement of the reliability.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2002-323108